1. Field of the Invention
This invention is concerned with an improved hydrophone for use in a hydrophone array deployed in a body of water. The hydrophone is insulated to prevent signal degradation due to conductive water pathways between hydrophone terminals due to water invasion.
2. Discussion of the Prior Art
In marine geophysical exploration, a ship tows a seismic streamer cable through a body of water along an assigned line of survey. The streamer cable may be 5,000 meters long and may contain many hundreds of hydrophones. At intervals, an acoustic wavefield is generated in the water. The acoustic wavefield travels downwardly and into the sub-water-bottom strata whence it is reflected back to the water surface where the reflected wavefield is detected by the hydrophones as electrical signals. The detected reflection signals are recorded for later analysis.
The seismic streamer cable typically includes a plurality of individual sections, each about 100 meters long. A section consists of a plastic tube or skin of polyurethane or the like, a few centimeters in diameter. Each end is terminated by a means for interconnecting the respective sections mechanically and electrically together to make up the full streamer cable. A section includes stress members, usually three, that extend from one section termination to the other, to absorb the towing forces. A plurality of hydrophones are emplaced along the section at intervals of a meter or less. The hydrophones are electrically interconnected into groups or arrays of ten or more sensors each, by color-coded conductors integrated into a wire bundle that runs along the length of the section, one pair of conductors per group. The section terminations include multi-contact electrical connector plugs so that signals from each group may be transmitted separately to a multi-channel recording device on the towing ship, one array per channel. The hydrophone signals are bi-polar, that is, one line is connected to a positive pole and the other line is connected to a negative pole of the hydrophone.
The section carcass, that is, the stress members, the wire bundle and the hydrophones are heavy. The cable section is therefore filled with a floatation fluid such as odorless kerosene so that the cable will be neutrally buoyant in the water. The fluid serves the additional purposes of electrically insulating the exposed hydrophone terminals and wire connections as well as providing a fluid medium for transmitting the hydrodynamic pressure variations, resulting from an acoustic pulse, from the surrounding water, to the hydrophones.
Typically, a hydrophone consists of a piezo-electric ceramic wafer cemented to each of a pair of thin metal diaphragms that are soldered or welded to the opposite ends of a short cylindrical metallic spacer to form a sealed case. The opposite surfaces of the wafers are silvered and the wafers are polarized. An electrical signal lead is soldered to one of the silvered surfaces. A feed-through, insulated terminal is mounted in the side of the cylindrical spacer, coupled to the signal lead, for electrical communication with the outside world. The other side of the wafer is cemented to the diaphragm by a conductive epoxy. The case forms one side and the feed-through terminal forms the other side of the bi-polar hydrophone circuit. Variations in the hydrodynamic pressures in the surrounding water due to transient acoustic wavefields flex the piezo-electric wafers. Flexation of a wafer generates an electric charge thereon to create an electric signal proportional to the pressure changes. See for example, U.S. Pat. Nos. 3,970,878, 4,336,639 and 4,517,664, all assigned to the assignee of this invention, for a description of typical construction methods and materials.
The hydrophones making up a hydrophone array are mounted mechanically in the streamer section at appropriate spacings. Electrical leads interconnect each one of an array of hydrophones, in parallel, to the two wires in the wire bundle that are assigned to service that array. That wire-pair serves as a signal trunk line for the array as a whole with the interconnecting leads between each hydrophone of the array and the wire bundle serving as branch lines. Typically, the connections of the branch lines to the trunk line are in the form of wire-wrapped or soldered Y connections.
So long as the integrity of the cable skin is preserved, the floatation fluid insulates the exposed electrical connections. Sometimes, sharks bite and puncture the cable skin or shrimp boats run over and gash the streamer cable which is usually deployed a few meters beneath the water surface. If the skin of a section is punctured, water invades the cable. Water, particularly salt water, provides a short-circuit pathway between the exposed hydrophone case and the feed-through terminal. Although the streamer cable may still be mechanically intact, it becomes electrically inert. It must be retracted back into the ship for repair or replacement of the damaged section. On the large seismic ships in current use, a shut-down merely due to electrical failure, costs many tens of thousands of dollars.
It is, of course, possible to cover the hydrophones and their exposed signal output terminals with a suitable potting compound. However, that material severely degrades the signal output strength and may even seriously distort the signal. Furthermore, the customary Y connections are very difficult to insulate against water seepage and water-wicking at the connections between the branch lines and the trunk line.
This invention provides a hydrophone whose terminals are completely isolated from one another and which are unaffected by water invasion; the invention provides a means for eliminating the troublesome Y connections to the trunk lines in the wire bundle. In the event of non-catastrophic mechanical damage to the streamer cable, electrical integrity is preserved without the need for immediate cable recovery for repairs.
In this disclosure, the term hydrostatic pressure refers to the pressure due to the weight of a column of water of a specified height. The term hydrodynamic pressure refers to variations in water pressure due to a transient acoustic wavefield.